1. Field of the Invention
The invention relates to fiber optic sensors. More particularly, the invention relates to methods and apparatus for mechanically enhancing the sensitivity of longitudinally loaded fiber optic sensors and for converting pressure or temperature to longitudinal strain on a fiber optic sensor.
2. State of the Art
Fiber optic sensor technology has developed concurrently with fiber optic telecommunication technology. The physical aspects of optical fibers which enable them to act as wave guides for light are affected by environmental influences such as temperature, pressure, and strain. These aspects of optical fibers which may be considered a disadvantage to the telecommunications industry are an important advantage to the fiber optic sensor industry.
Optical fibers, whether used in telecommunications or as environmental sensors, generally include a cylindrical core, a concentric cylindrical cladding surrounding the core, and a concentric cylindrical protective jacket or buffer surrounding the cladding. The core is made of transparent glass or plastic having a certain index of refraction. The cladding is also made of transparent glass or plastic, but having a different, smaller, index of refraction. The ability of the optical fiber to act as a bendable waveguide is largely determined by the relative refractive indices of the core and the cladding.
The refractive index of a transparent medium is the ratio of the velocity of light in a vacuum to the velocity of light in the medium. As a beam of light enters a medium, the change in velocity causes the beam to change direction. More specifically, as a beam of light travels from one medium into another medium, the beam changes direction at the interface of the two media. In addition to changing direction at the interface of two media, a portion of the incident beam is reflected at the interface such that the energy of the beam travelling through the second medium is diminished (the sum of the energy of the refracted and reflected beams must equal the energy of the incident beam). The angles of reflection and refraction can be predicted using Snell's law if the refractive indices of both media are known.
By altering the indices of refraction of two adjacent media, the angle of refraction and the angle of reflection of a beam travelling toward the interface of the two media can be altered such that the intensity of the light entering the second medium approaches zero and substantially all of the light is reflected at the interface. Conversely, for any two transparent media, there is a critical angle of incidence at their interface at or below which substantially all of the incident light will be reflected. This phenomenon, known as total internal reflection, is applied in choosing the refractive indices of the core and the cladding in optical fibers so that light may propagate through the core of the fiber with minimal power loss.
As mentioned above, fiber optic sensors employ the fact that environmental effects can alter the amplitude, phase, frequency, spectral content, or polarization of light propagated through an optical fiber. The primary advantages of fiber optic sensors include their ability to be light weight, very small, passive, energy efficient, rugged, and immune to electromagnetic interference. In addition, fiber optic sensors have the potential for very high sensitivity, large dynamic range, and wide bandwidth. Further, a certain class of fiber sensors may be distributed or multiplexed along a length of fiber. They may also be embedded into materials.
State of the art fiber optic sensors can be classified as either "extrinsic" or "intrinsic". Extrinsic sensors rely on some other device being coupled to the fiber optic in order to translate environmental effects into changes in the properties of the light in the fiber optic. Intrinsic sensors rely only on the properties of the optical fiber in order to measure ambient environmental effects. Known fiber optic sensors include linear position sensors, rotational position sensors, fluid level sensors, temperature sensors, strain gauges, fiber optic gyroscopes, and pressure sensors.
One type of fiber optic sensor utilizes intra-core fiber gratings. Intra-core Bragg gratings are formed in a fiber optic by doping an optical fiber with material such as germania and then exposing the side of the fiber to an interference pattern to produce sinusoidal variations in the refractive index of the core. Two presently known methods of providing the interference pattern are by holographic imaging and by phase mask grating. Holographic imaging utilizes two short wavelength (usually 240 nm) laser beams which are imaged through the side of a fiber core to form the interference pattern. The bright fringes of the interference pattern cause the index of refraction of the core to be "modulated" resulting in the formation of a fiber grating. Similar results are obtained using short pulses of laser light, writing fiber gratings line by line through the use of phase masks. By adjusting the fringe spacing of the interference pattern, the periodic index of refraction can be varied as desired.
It has been demonstrated that an ultrahigh hydrostatic pressure induces fractional changes in the physical length of a fiber optic and thus induces a fractional change in the Bragg wavelength of a grating incorporated in the fiber core. For example, M. G. Xu et al., Optical In-Fibre Grating High Pressure Sensor, Electron. Lett., Vol. 29, No. 4, pp. 398-399 (1993), demonstrates how a fiber optic Bragg grating sensor can be used to measure very high pressure. In particular, the Xu et al. paper demonstrates a simple in-fiber grating sensor which exhibits a linear Bragg wavelength shift of 3.04.times.10.sup.-3 nm/MPa. The authors note that the sensor is also sensitive to changes in temperature. They note a linear Bragg wavelength shift of 10.45.times.10.sup.-3 nm/.degree. C. and specifically state that far more compensation for the effects of temperature is necessary for their sensor to be valuable as a pressure sensor and that the real advantage of their sensor is only evident at ultrahigh pressure.
It has been suggested that a mechanical structure be attached to a Bragg grating sensor in order to enhance its sensitivity to pressure. For example, M. G. Xu et al., Fibre Grating Pressure Sensor with Enhanced Sensitivity Using a Glass-Bubble Housing, Electron. Lett., Vol. 32, No. 2, pp. 128-129 (1993), demonstrates how pressure sensitivity is enhanced by housing the fiber with Bragg grating in a glass bubble. When the glass bubble is pressurized, the fractional change in the diameter of the glass bubble .DELTA.d/d owing to a pressure change .DELTA.P is given by Equation 1 where E is the Youngs modulus of the bubble, .mu. is the Poisson ratio of the bubble, and t is the wall thickness of the bubble. ##EQU1##
If there is good bonding between the fiber and the glass bubble, the pressure induced strain on the grating is equal to the fractional change in the diameter of the glass bubble .DELTA.d/d. The pressure sensitivity, defined as the fractional change in the Bragg wavelength .DELTA..lambda..sub.B /.lambda..sub.B is given by Equation 2 where P.sub.e =0.22 is the effective photoelastic constant for silica. ##EQU2##
The glass bubble increased pressure sensitivity of the Bragg grating by a factor of four. It would seem, however, that the glass bubble structure would not be suitable for use in harsh environments.
WO 98/31987 to Maron et al. discloses a multiparameter fiber optic sensor for use in harsh environments such as in the borehole of an oil well. The sensor generally includes a fiber optic having three or four spaced apart Bragg gratings all mounted in a single capillary tube with a diaphragm bonded to one end of the capillary tube. Various materials are located between the fiber optic and the capillary tube along the length of the capillary tube and adjacent the Bragg gratings. The three or four spaced apart Bragg gratings provide a pressure sensor, an acceleration (or vibration) sensor, and a temperature sensor. Each of the sensors is isolated from the other sensors by "rigid elements" located between the fiber optic and the capillary tube. The pressure sensor is activated by the diaphragm at the end of the capillary tube which causes material surrounding the closest Bragg grating to place an axial strain on the Bragg grating. The acceleration sensor is activated by a free moving mass which impacts a rigid member adjacent to the next Bragg grating and axially strains the grating in proportion to the acceleration of the mass. The temperature sensor(s) are formed by one or two Bragg gratings adjacent one or two rigid members near the end of the tube opposite the end having the diaphragm. One of the disadvantages of the multiparamter sensor described by Maron et al. is that the pressure sensor must be located at the end of the device with a diaphragm arranged orthogonal to the end of the fiber optic. This prevents the arrangement of several pressure sensors along a single fiber optic unless beam splitters are used to branch out the fiber. As mentioned above, one of the inherent advantages of Bragg grating fiber optic sensors is that many sensors may be arranged along a long length of single fiber through the use of wavelength or time division multiplexing.